Classical Dynamics of The Benzene Molecule

These animations depict studies of the classical vibrational dynamics of benzene.
The molecular dynamics were computed using the most extensive ab initio
force field for a polyatomic molecule from Maslen, Handy, Amos, and Jayatilaka
1.
This force field contains fourth-order terms supplemented by fifth- and
sixth-order terms by Wyatt and Iung2.
The primitive direct product space has dimension 36.5 billion, which was
reduced to an active space of 16,000 by use of the Wave Operator Sorting
Algorithm3.

The study follows many possible trajectories of the molecule after excitation.
(By the term trajectory, we refer to the (adiabatic) distribution of
energy in a 21- or 30-dimensional space.)
The trajectory of the molecule is determined by the initial conditions (potential
and kinetic energy in each normal mode, corresponding to the position and
momentum of each atom) and by various conditions imposed on the model.
Each simulation begins by assigning a zero-point potential and kinetic
energy value to each normal mode in the molecule;
the proportion and phase are randomly selected.
The next step is to add energy to the appropriate C-H stretch mode.
From that point, the simulation follows the energetics of each mode of the
21- or 30-mode system.
A large number of these cases are not physically viable (e.g. lead to
a negative energy in one of the modes or lead to energy in a mode that exceeds
the bond energy), and these cases are discarded.
The remaining trajectories represent physically significant configurations.
Each animation follows the course of one such trajectory.

Data from three different simulations are displayed.
In the first animation, the motion of the molecule is constrained to a plane
(i.e., 21-mode benzene4).
In the other two animations, out-of-plane motion is allowed (i.e., 30-mode
benzene5,6).
The total duration of each of the full simulations is 2.4 picoseconds, though the
short example animations below represent only small fractions of this interval.
The interval of each time step is 200 a.u. (4.8 fs) for the studies.

In these animations, the small blue spheres denote the hydrogen atoms, and the red
spheres denotes the carbon atoms.

The computations were performed by Todd J. Minehardt and
Robert E. Wyatt
using
lonestar, a
Cray T3E
Massively Parallel Computer system.
The computation required about 30 hours of dedicated time using 32 nodes of
lonestar;
the total computational time would be about 1,000 hours if the program were
run on a single-processor computer.
These animations were produced by
J. David Adcock,
a graduate student in the
Wyatt
research group.
The graphics were rendered using the
POV Ray Version 3.0.1
program.
For more information, send e-mail to
david@quantum.cm.utexas.edu.